This invention relates to a molded resin semiconductor device having a plurality of semiconductor chips mounted in a single case and resin encapsulated.
Recently, semiconductor power modules of this type have become popular and their markets are rapidly expanding. The chips in such modules are connected in a double-arm configuration, a double-star configuration, a bridge configuration and the like.
Semiconductor power modules have the advantages that a plurality of them can be mounted on a single heat radiation fin together with other electrical elements, and they are convenient to use. However, stresses from thermal expansion and contraction due to the heat generated at the surfaces of the plural chips potted in thermosetting epoxy resin are greater than the stresses in conventional molded resin semiconductor elements having just a single chip.
Strain occurs at the points of contact between the resin and the copper leads or radiation plate due to the difference between the coefficients of thermal expansion of the resin and the metal, and the stress generated thereby is concentrated on those chips which have the weakest mechanical strength, which leads to damage.
To solve this problem the structure shown in FIG. 1 has been proposed, wherein a case 2 which may, for example, be rectangular, has one end mounted on the upper surface of a radiation base plate 1 of copper. An insulating plate 3 is fixed to the surface of the plate 1. A first L-shaped terminal plate 4 has a vertical portion 4a and a horizontal portion 4b fixed to the insulating plate 3.
First and second semiconducting chips 5, 6 which function as diodes or thyristors are fixed on the horizontal portion 4b, which connects one electrode of each chip to the first terminal plate 4. One end of first and second lead wires 7, 8 is connected to the other electrode of the first and second chips 5, 6. The lead wires 7, 8 are elastic copper ribbons.
A first flexible resin 9 such as gelled silicone or the like injected into the case 2 covers the insulating plate 3, the horizontal portion 4b of the first terminal plate, the chips 5, 6, and the lead wires 7, 8. The vertical portion 4a of the terminal plate 4 projects out from the first resin 9, and both upper ends of the lead wires 7, 8 are exposed at the surface thereof. Second and third terminal plates 10, 11 are connected to the exposed ends of the lead wires 7, 8.
A second thermosetting resin 12 such as epoxy or the like is applied over the first resin 9 in the case 2, with the other ends of the first, second and third terminal plates 4, 10, and 11 projecting outwardly therefrom.
When the device is operating, current intermittently flowing through the semiconductor chips 5, 6 causes strain in the base plate 1 and the second resin 12 resulting from the differences in their coefficients of thermal expansion. The stress generated by this strain is isolated from the chips 5, 6 by the flexibility of the first encapsulating resin 9 and its disposition between the plate 1 and the resin 12. The flexibility of the resin 9 also complements the elasticity of the lead wires 7, 8 since any stress applied to or generated by such wires is effectively isolated from the semiconductor chips.
Molded semiconductor devices like the one shown in FIG. 1 are disadvantageous, however, in that the coefficient of thermal expansion of the gelled silicone resin 9 is greater than that of the epoxy resin 12, and the thermal conductivity of the gelled silicone is comparatively small. Therefore, when intermittent current flows through the chips the heat generated thereby will flow through the base plate 1, the terminal plate 4, and the insulating plate 3, but very little flows through the resin 9 due to its lower thermal conductivity. As a result, the circumferences of the chips become hotter than the resin 9 near its interface with the resin 12, and the difference in thermal expansion becomes greater between the chips and the resin 9 near the chips and than between the resin 9 and the resin 12 near their interface.
The circumferences of the chips and the lead wires 7, 8 are thus exposed to stress due to such differences in thermal expansion, which fatigues the solder connecting the chips with the terminal plate 4 and the lead wires. As a result, such molded semiconductor devices do not consistently maintain their operating characteristics over a long period of time.